It has been shown that unattached kinetochores produce a "wait anaphase" signal in vertebrate cells until the last chromosome becomes bi-oriented. However, the nature of this signal is still unknown. One useful model for this problem is multinucleated cells obtained by means of cell fusion. A chimeric cell can be created with two separated spindles in one cytoplasm. Such a model allows one to study how unattached chromosomes in one spindle affect anaphase onset in the other spindle. If the "wait anaphase" factor is diffusible it should spread throughout the cytoplasm and affect all spindles in the cell. By contrast, if the factor is mechanical, and somehow related to the positions of the chromosomes along the spindle or the shape of the spindle itself, the presence of an extra spindle in the cell should not affect the other spindle. If the factor is isolated to structural elements in the cell, the distribution of those elements in relation to the spindles should influence anaphase onset. To date Ms. Paliulis, a student at Williams College (MA), is fuising PtK1 cells and follow the mitotic divisions in the cells. The data currently suggest that separate spindles start anaphase independently e.g. the wait anaphase signal is restricted to the spindle itself.